Abstract
AbstractAb initio studies on the SN2 identity exchange reactions RCH2X + X− → X− + RCH2X for R = CH2CH with X− = H, NH2, OH, F, PH2, SH and Cl, and for R = CH3 and CHC with X− = Cl were carried out at the HF and MP2 levels using the 6–31 ++ G** basis sets. The activation barriers, ΔE≠, and major structural changes, Δd≠ (C–X), in the activation process are closely related to the electronegativity of the R and X groups. The effect of electronegativity is twofold: a stronger electronegativity of R and/or X leads to a lesser electronic as well as structural reorganization required in the activation and to a greater correlation energy in the transition state. The former effect lowers the energy barriers at both the HF and MP2 levels whereas the latter lowers only the correlated (MP2) activation energies. Results with R = CH2CH, as a model for R = C6H5, indicate that ‘benzylic effect’ arises mainly from the relatively stronger electron acceptor ability of the phenyl group.
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